Aside from purely mechanical methods, the prior art shows three methods for the joining of titanium base articles: welding, brazing, and diffusion bonding. A fourth method, liquid interface diffusion (LID) bonding, may be characterized as a hybrid of brazing and diffusion bonding. Welding, brazing, and LID bonding each use a filler metal to bridge the joint between the articles.
One brazing filler metal composition for brazing titanium base articles is described in U.S. Pat. No. 3,652,237. The filler is fabricated as a foil, and has a nominal composition of, by weight percent, Ti-15Ni-15Cu. The filler has a composite structure, characterized by one or more layers each of copper and nickel surrounded by an outer layer of titanium. The foil suffers from two significant problems: first, when the filler melts, the liquid must penetrate inevitable layers of titanium oxide on the foil surfaces and on the faying surface of each article. Such oxide barriers are an impediment to optimum joint formation. Second, it has been found that the distance between the faying surfaces must be small in order to avoid the formation of voids or cracks in the braze joint. Such discontinuities are apparently caused by an inadequate supply of liquid filler metal to the gap between the articles being joined.
Other braze foils are described in U.S. Pat. Nos. 4,026,677, 4,034,454, and 4,034,906. These filler metals are, generally, Ti-50Zr foils having a layer of Cu or layers of Cu and Be. Due to the toxicity of Be, its use is discouraged. The low melting point of the Ti-Zr-Cu foils precludes their use in some applications.
Various braze alloys for joining honeycomb to a facing sheet are described in U.S. Pat. No. 3,683,488. Layers of metal such as Au, Ni, Ag, and Cu are electroplated onto the honeycomb cell walls. However, plating onto honeycomb is difficult, and engineers have sought improved methods for brazing.
Filler metals for use in the LID bonding of titanium have been the subject of numerous patents. See, e.g., U.S. Pat. Nos. 3768,985, 3,769,101, 3,854,194, 3,981,429, 4,029,479, and 4,318,965. Each of these patents describes a Ni-Ag-Cu filler metal system wherein the copper and nickel are present in equal amounts. Layers of the three elements are deposited onto the faying surface of one of the parts to be joined, or layers are deposited onto a very thin titanium foil which is then disposed between the parts. The total thickness of the electroplated foil is about 15 microns. The thin filler is capable of providing only a small amount of liquid to the joint, and due to rapid diffusion which occurs during the bonding process, the filler is liquid for only a short period of time (see U.S. Pat. No. 3,854,194 at column 5, line 37). Such rapid diffusion may preclude adequate joint formation. Additionally, the favored LID bonding temperature for these filler metal systems is about 970.degree. C. (see U.S. Pat. No. 3,981,429 at column 3, line 7) which may preclude their use in joining some titanium alloys. Furthermore, titanium base articles joined with filler metals containing silver have been observed to fracture prematurely during service; these fractures appear to initiate at Ti-Ag intermetallics which form during the bonding process.
Layers of copper, nickel, and copper are sequentially plated onto the cell walls of titanium honeycomb for LID bonding according to U.S. Pat. No. 3,957,194. As noted above, plating on honeycomb is undesired.
Notwithstanding the availability of the brazing and LID bonding filler metals used to join titanium base substrates described above, each suffers from one or more drawbacks which limits its use. Consequently, researchers are continually striving to improve upon the existing state of the art. Specifically, they seek to define an improved filler metal for use in the brazing of titanium base articles, wherein the melting and solidification characteristics of the filler metal are such that wide gaps between the faying surfaces are tolerated, while at the same time, the braze joint has tensile properties comparable to those of the parent material.